The conventional V belt stepless variable transmission constructed as foregoing has hitheto been well-known, which, as disclosed in the Japanese Patent Laid-Open Gazette No. Sho 58-42,862, hydraulically controls the movable conical disc at the driving pulley cooresponding to the number or rotations of an engine and the degree of opening of a throttle, and the movable conical disc at the driven pulley for applying a predetermined tension to the V belt.
In this case, since the tension applied to the V belt is controlled corresponding to a pulley ratio and the number of rotations of the engine, the V belt is not subjected to tension to meet input torque, thereby creating the defect of reducing a life span of the V belt and deteriorating the transmission efficiency. For eliminating such defect, it is proposed that the movable conical disc is urged by an urging force corresponding to the input torque so that belt tension to meet it is generated. Such method has to convert the detected input torque into hydraulic pressure and then the hydraulic pressure into the urging force to be applied to the movable conical disc, thereby creating the problem in that such multistage conversion process complicates the transmission and variation in oil temperature causes errors that are too large.
Another stepless variable transmission has been disclosed in the Japanese Patent Laid-Open Gazette No. Sho 58-142,060, which is provided at the driving pulley with a weight roller using a centrifugal force for axially urging the movable conical disc at the same, at the driven pulley with a torque cam for axially urging the movable conical disc at the driven pulley corresponding to the transmission torque, and at both the driving and driven pulleys with mechanical control mechanisms, thereby designing the transmission simple in construction.
In this case, since the torque cam can obtain the linear urging force from the transmission torque to the movable conical disc, it is advantageous that proper tension to meet the transmission torque can be applied to the V belt, but the driving pulley, which employs a centrifugal weight roller, is controllable only of the pulley ratio corresponding to the number of rotations of the engine. Hence, the latter method is inferior to the former method in accuracy of the hydraulic control. Also, since the movable conical disc is urged by a force proportional to the square of the number of rotations of the engine, the V belt is more loaded than it needs, resulting in the defect not only of reducing the life span of the V belt but also of causing a larger power loss.